The calicheamicin [Lee et al., J. Am. Chem. Soc., 109:3466 (1987)] and esperamicin [Golik et al., J. Am. Chem. Soc., 109:3462 (1987)] families of antibiotics contain a complex bicyclic enediyne allylic trisulfide core structure linked through glycosyl bonds to an oligosaccharide chain. The oligosaccharide portions of each of those molecules contain a number of substituted sugar derivatives, and each of those oligosaccharide portions contains a tetrahydropyran ring that is substituted on the ring both with a sulfur atom and with the oxygen atom of a hydroxylamine group.
The chemical structure of calicheamicin .gamma..sub.1.sup.I, which contains a more complex oligosaccharide group than an esperamicin, is illustrated in FIG. 1 herein. The saccharide unit lettered "B" is the before discussed sulfur- and O-hydroxylamine-substituted tetrahydropyran derivative.
The saccharide rings of an esperamicin corresponding to rings "A" and "E" of FIG. 1 are substituted similarly to those depicted, except that the esperamicin ring corresponding to ring E includes an N-isopropyl rather than N-ethyl group. The corresponding "B" ring of an esperamicin contains an S-methyl rather than the S-(saccharide-substituted)-derivatized benzoyl group (C and D rings) shown in FIG. 1. The structures of esperamicin and some of its derivatives are illustrated in U.S. Pat. No. 4,837,206, whose disclosures are incorporated by reference.
The enediyne-containing (aglycone or core) and carbohydrate portions of calicheamicin and esperamicin appear to carry out different roles in the biological activity of those molecules. Thus, the core portion appears to cleave DNA [Zein et al., Science, 240:1198 (1988)], whereas the oligosaccharide portion of calicheamicin appears to guide the drug to a double stranded DNA minor groove in which the drug anchors itself on the 5' side of a TCCT sequence, and the core cleaves the DNA. Esperamicins are less sequence specific. [Zein et al., Science, 244:697 (1989)].
Studies of the effect on DNA cleavage of derivatization or removal of one or more of the D and E rings of calicheamicin (FIG. 1) indicate the following: removal of the E ring (amino sugar) provided a drug with the same DNA cleaving specificity as the parent, but having a DNA-cleaving efficiency 2 to 3 orders of magnitude less; acylation of the E ring amine maintained specificity but lowered efficiency; removal of the D ring (terminal rhamnose) maintained specificity, but lowered efficiency 50-100 times; and removal of the D and E rings (terminal rhamnose and amino sugar) resulted in inhibition of cutting. [Zein et al., Science, 244:697 (1989)].
Esperamicin lacks the C and D rings and includes a further complex saccharide structure linked to an additional core hydroxyl group. U.K. Patent Application 2,179,649A reports that acid hydrolysis of esperamicins led to cleavage of that second complex saccharide structure and a resulting esperamicin derivative referred to as BBM-1675C that was about as effective as the starting esperamicin BBM-1675A.sub.1 (esperamicin A.sub.1), and more so than esperamicin BBM-1675A.sub.2 (esperamicin A.sub.2) as an antitumor and antimicrobial agent. From the discussion in this U.K. application, the oligosaccharide portion of BBM-1675C contains rings analogous to the A, B and E rings of calicheamicin shown in FIG. 1.
U.K. Patent Application 2,179,649A also disclosed that further hydrolysis of esperamicin BBM-1675C led to another esperamicin derivative named BBM-1675D that was also said to be about as effective as esperamicin BBM-1675A.sub.1, as an antitumor and antimicrobial agent. The data presented indicate that esperamicin BBM-1675D possessed only two saccharide rings; i.e. those corresponding to the A and E rings of FIG. 1 herein.
Thus, the art has recognized the importance of the oligosaccharide portions of the calicheamicin and esperamicin antibiotics, and has recognized that the saccharide rings in the calicheamicin group can affect the activity of the drug. The results disclosed in Zein et al., Science, 244:697 (1989) and those in U.K. Patent Application 2,179,649A indicate a possible conflict as to the effect of the individual saccharide portions on efficacy, although different assay methods were used.
It would be important therefore to be able to prepare an oligosaccharide portion of a calicheamicin or the corresponding portion of an esperamicin and derivatives thereof so that the specificities of those materials can be further studied and fine-tuned. It would also be of importance to link a calicheamicin or esperamicin oligosaccharide or a derivative or analog thereof to another known DNA cleaving chemical to create a synthetic, chimeric antibiotic.
The present invention describes the synthesis of key intermediates useful in the preparation of a calicheamicin or esperamicin oligosaccharide portion, an oligosaccharide portion derivative or analog, as well as the synthesis of chimeric antibiotics containing such an oligosaccharide.